test(datetime): test each part type in dateTimeFormatter.format

[WWRS]

Previously, larger tests meant these appeared to be covered, but there was no testing of the deep specifications for each part type. Covering the cases revealed a bug with fractionalSecond, and the fix is included in this PR.

I also made formatToParts require exactly 2 digits when parsing yy and exactly 4 digits when parsing yyyy instead of 1-2 or 1-4 respectively. The spec requires yy to represent exactly 2 digits, but yyyy technically represents 4+ digits. In neither case are fewer digits allowed. I think it's unlikely anyone will be trying to parse years with 5+ digits, and parsing yyyyMMdd would require a more sophisticated approach if we allowed 5+, so I think only allowing 4 is best.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.30%. Comparing base (96116ce) to head (f086791).
Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6516      +/-   ##
==========================================
- Coverage   95.50%   95.30%   -0.21%     
==========================================
  Files         575      576       +1     
  Lines       43279    43326      +47     
  Branches     6472     6483      +11     
==========================================
- Hits        41335    41290      -45     
- Misses       1905     1995      +90     
- Partials       39       41       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.